How to make my animation smooth like a curtain falling down
Im not sure for the result ... not clear what it should represent in reality - material, dimension, if it use some metal (or other hard) rings etc.)
Example is just

model scaled in Z axis (to more fit final shape)
Cloth - Preset Denim with Quality 4, Mass 0,8 kg
Collision Quality 8 and enabled Self Collision
first 50 frames used to squeeze cloth up by another Plane collider